This invention relates to estimation of Doppler shift for, and recovery of a carrier signal for, a received signal.
In a traditional satellite communication system, the satellites are located in a geo-stationary orbit, approximately 35,784 km from the center of the Earth, and move at the same angular velocity as the Earth""s surface, approximately 7.292xc3x9710xe2x88x925 radians per second. The Doppler frequency offset for a signal received from a transmitter located on such a satellite is small, usually much less than the symbol rate for the received signal. This small frequency offset reduces the requirements for carrier signal recovery at the receiver, assumed to be located on or near the Earth""s surface. However, because of the great distance between a geo-stationary (GEO) satellite and a ground receiver, the signal round trip time propagation time is relatively long, at least about 0.1 sec, and such a transmission system offers a correspondingly reduced data transmission rate and requires relatively high transmitter power.
An alternative to the GEO system, referred to as a low Earth orbit or LEO system, locates one or more transmitting satellites closer to the Earth""s surface, for example, at a distance of about 350 miles above the surface, and the satellites move with a greater angular velocity than does a GEO satellite, the corresponding Doppler frequency offset is higher, the round trip signal propagation time is reduced, the permitted data transmission rate is increased, and the required transmission power is reduced. A LEO satellite is moving faster than the in-view receiver, and the larger Doppler frequency offset is often larger than the symbol rate for the received signal. Estimation of the Doppler frequency offset and recovery of the carrier signal is a challenging task and usually requires a more complex receiver design.
Automatic frequency control (AFC), phase locked loop (PLL) processing, fast Fourier transform (FFT) processing and linear prediction (LP) are possible candidates for processing a Doppler-shifted received signal. AFC and/or PLL are presently used for cellular communications with GEO satellites, where the Doppler shift is at most a few hundred Hz. The maximum Doppler frequency offset that can be corrected using AFC or PLL is about 10 percent of the symbol rate, and the symbol rate is likely to be as low as 10-50 KHz. Further, the frequency acquisition time for AFC and/or PLL will depend strongly on the signal-to-noise ratio (SNR) or bit energy-to-noise ratio (Eb/No) and is generally much longer than the available time interval (e.g., a time slot length) when the Doppler frequency offset is large. Thus, use of AFC and/or PLL alone will not allow fast or reliable acquisition of a Doppler-shifted signal.
An FFT approach can be used to assist in carrier recovery by estimating a large frequency offset in a fixed time period. However, an FFT approach is complex, requires performance of a set of computations that is approximately proportional to Nxc2x7log(N), where N is the number of signal samples used for the estimates, and must operate in a block mode for computations so that all samples must be collected before computations begin. This will require complex processing and will not allow estimation of a Doppler frequency offset within a short time interval.
An LP approach can also be used to assist in carrier recovery by estimating a large frequency offset in a fixed time period. LP requires performance of a set of LP computations that is approximately proportional to N and operates in serial mode so that computations can begin before all signal samples are collected. However, the accuracy of an LP approach depends upon the frequency estimation range (larger ranges produce poorer accuracy) and upon the SNR. For medium to low SNR with a frequency estimation range larger than the symbol rate, an LP approach cannot estimate frequency offset to better than to within 5 percent of symbol rate with high probability.
What is needed is a system for Doppler frequency offset estimation and carrier signal recovery that has relatively low complexity and that provides an estimate within a time interval allotted to transmission of a few symbols in a time slot (a time interval enclosing one information unit, including a preamble, a unique word or other identifying indicium, and a payload). Preferably, the system should be able to estimate a Doppler frequency offset of any size, even one that is greater than the symbol transmission rate, should have a relatively short signal interrogation time for such estimation, should be accurate to within one percent for a received signal interrogated within an assigned time slot, should work quickly to provide a Doppler frequency offset estimate before the information unit has been completely received, should work with a bit energy ratio Eb/No as low as 4.5 dB, and should have relatively low complexity.
These needs are met by the invention, which provides a simple, multi-stage Doppler frequency offset estimation system that (1) provides an estimate of Doppler frequency offset within one percent of the correct value, (2) provides this estimate within the present time slot and with relatively simple computations. The system uses first and second stage LP analysis with different parameter sets chosen for each of these stages, followed by a decision feedback PLL third stage that acquires and subsequently tracks the Doppler-shifted received signal. The first two stages provide down-conversion of the estimated Doppler frequency offset to a residual shift that can be captured and tracked by the PLL. The third stage uses decision feedback, second order PLL to acquire and track the residual frequency offset and phase angle. The final Doppler frequency offset is calculated from the results of all three stages, which operate serially and continuously. The system is especially useful for receipt of low orbit satellite signals with small to medium SNR.